Consider the hamburger. Producing this staple of the U.S. diet takes 25 kilograms of animal feed, 25 square meters of land, and about 220 liters of water—all for four patties. Statistics like those have persuaded some scientists and environmental activists that eating less meat could have a big impact on carbon and other greenhouse gas emissions. But what would happen if every American made the switch to a plant-only diet? According to a new study, a nation of 320 million vegans would reduce greenhouse gas emissions from agriculture by some 28%, far less than the amount now produced by the livestock industry. The authors claim the switch could also lead to deficiencies in key nutrients—including calcium and several vitamins.